This invention relates generally to sulfur recovery processes and more particularly to an improved Claus process and to a process for cleanup of sulfur-containing constituents in a gaseous stream such as a tail gas from a sulfur recovery unit (SRU).
The recovery of sulfur and cleanup of sulfur-containing constituents or compounds in a gaseous stream may be economically and/or environmentally motivated. The Claus process is widely used for the recovery and production of elemental sulfur from acid or sour gas streams also containing hydrogen sulfide in admixture with carbon dioxide. Additional sulfur-containing compounds which may be present or otherwise encountered or formed in the process include sulfur dioxide, carbonyl sulfide and carbon disulfide. The acid gas streams usually also contain small amounts of hydrocarbons ranging from methane to butane and even higher molecular weight hydrocarbons.
In most Claus processes, an initial thermal reaction zone or reactor is used with the addition of an oxidant such as air to react hydrogen sulfide to sulfur. In addition, sulfur dioxide and water are formed. Following separation of the sulfur product, the gases from the thermal reaction zone are passed through one or more catalytic reactors or stages wherein sulfur dioxide is reacted with hydrogen sulfide over alumina or bauxite catalysts to produce additional sulfur. As the inert content of the acid gas stream increases, the thermal reaction becomes unstable and preheating or diverting of a portion of the acid gas feed around the thermal reactor is required in modified Claus processes. Thus, the conversion of contaminants to sulfur and the percent of sulfur recovery in such processes are limited by the Claus reaction thermodynamics or kinetics.
The Claus process is sensitive to variations in the feed rate of the acid gas stream and relatively minor variations in upstream units tend to amplify in the Claus plant, "Claus Processing of Novel Acid Gas Streams", Beavon, David K., Symposium on Sulfur Recovery and Utilization, presented before the division of Petroleum Chemistry, Inc., American Chemical Society, Atlanta meeting, Mar. 29-Apr. 3, 1981. In acid gas streams containing relatively high proportions of inerts, e.g. carbon dioxide, the bypass of the thermal reactor is prohibited when even traces of olefins or aromatics are present since they react to form tarry products which foul the catalyst and discolor the sulfur, Beavon, supra.
The Claus process as described above removes about 93-96% of the originally present sulfur and generally requires an accompanying tail gas cleanup process and plant process equipment to remove the unreacted sulfurous compounds, primarily sulfur dioxide and hydrogen sulfide together with lesser amounts of carbonyl sulfide and carbon disulfide. Known tail gas cleanup techniques may increase the overall recovery to 99.5-99.6% without meeting recent emission standards in the range of 100 ppm and less. The costs of such tail gas cleanup plants often exceed the cost of the Claus plant for bulk sulfur recovery, and therefore the plants may be based on emission standards rather than economics.
Considerable effort has been expended to develop economical and effective tail gas cleanup processes. An early survey of prior processes is provided in "Current Claus Tail Gas Cleanup Processes", by B. Gene Goar, as presented at the Fifty Seventh Annual Convention of the Gas Processors Association, 1978. This survey is updated in "Emerging New Sulfur Recovery Technologies", by B. Gene Goar, Proceedings of the Gas Conditioning Conference, page 87, Mar. 6-8, 1989. Certain of these processes include conversion of the sulfur-containing compounds in the tail gas to a single species prior to absorption thereof. The advantages of conversion to a single sulfur species prior to water removal is discussed in U.S. Pat. Nos. 3,752,877 and 4,426,369. The Shell Claus Offgas Treating (SCOT) process is considered to be one of the most flexible processes available. In such process, all of the sulfurous compounds are converted to hydrogen sulfide and in a final stage selectively absorbed with an alkanolamine solution. The purified tail gas for venting contains about 200-500 ppm hydrogen sulfide which must be incinerated to sulfur dioxide before venting. This exceeds some current emission standards and may restrict use of the process. As reported by Goar, supra, the captial cost of the SCOT plant may equal the cost of the parent Claus plant in an "add-on" application and equals about 75-80% of the Claus cost in an integrated new plant.
Pending patent application U.S. Ser. No. 234,864, filed Aug. 19, 1988, which is owned by the assignee herein, discloses an improved tail gas cleanup process wherein only a single thermal or catalytic reactor or reaction stage is used in a Claus reaction. The relatively high concentrations of unreacted The relatively high concentrations of unreacted hydrogen sulfide and sulfur dioxide resulting from the use of a single reactor stage are recovered with conversion to sulfur dioxide by incineration and concentrated by absorption and crystallization processes for recycle in such process. More particularly, a liquid carbon dioxide absorbent is used to remove contaminant sulfur-containing components which are thereafter separated for recycle. In this manner, substantially all sulfur-containing compounds are removed from the tail gas to provide sulfur dioxide for use in the Claus reaction. The sulfurous compounds in the feed stream to the Claus reactor are concentrated by crystallization or distillation prior to or following combination with the recycle stream.